Gene therapy holds enormous potential for a new era of human therapeutics. These methodologies will allow treatment for conditions that have not been addressable by standard medical practice. Gene therapy can include the many variations of genome editing techniques such as disruption or correction of a gene locus, and insertion of an expressible transgene that can be controlled either by a specific exogenous promoter fused to the transgene, or by the endogenous promoter found at the site of insertion into the genome.
Delivery and insertion of the transgene are examples of hurdles that must be solved for any real implementation of this technology. For example, although a variety of gene delivery methods are potentially available for therapeutic use, all involve substantial tradeoffs between safety, durability and level of expression. Methods that provide the transgene as an episome (e.g. basic adenovirus, AAV and plasmid-based systems) are generally safe and can yield high initial expression levels, however, these methods lack robust episome replication, which may limit the duration of expression in mitotically active tissues. In contrast, delivery methods that result in the random integration of the desired transgene (e.g. integrating lentivirus) provide more durable expression but, due to the untargeted nature of the random insertion, may provoke unregulated growth in the recipient cells, potentially leading to malignancy via activation of oncogenes in the vicinity of the randomly integrated transgene cassette. Moreover, although transgene integration avoids replication-driven loss, it does not prevent eventual silencing of the exogenous promoter fused to the transgene. Over time, such silencing results in reduced transgene expression for the majority of random insertion events. In addition, integration of a transgene rarely occurs in every target cell, which can make it difficult to achieve a high enough expression level of the transgene of interest to achieve the desired therapeutic effect.
In recent years, a new strategy for transgene integration has been developed that uses cleavage with site-specific nucleases to bias insertion into a chosen genomic locus (see, e.g., co-owned U.S. Pat. No. 7,888,121). This approach offers the prospect of improved transgene expression, increased safety and expressional durability, as compared to classic integration approaches, since it allows exact transgene positioning for a minimal risk of gene silencing or activation of nearby oncogenes.
One approach involves the integration of a transgene into its cognate locus, for example, insertion of a wild type transgene into the endogenous locus to correct a mutant gene. Alternatively, the transgene may be inserted into a non-cognate locus chosen specifically for its beneficial properties. See, e.g., U.S. Patent Publication No. 20120128635 relating to targeted insertion of a factor IX (FIX) transgene. Targeting the cognate locus can be useful if one wishes to replace expression of the endogenous gene with the transgene while still maintaining the expressional control exerted by the endogenous regulatory elements. Specific nucleases can be used that cleave within or near the endogenous locus and the transgene can be integrated at the site of cleavage through homology directed repair (HDR) or by end capture during non-homologous end joining (NHEJ). The integration process is determined by the use or non-use of regions of homology in the transgene donors between the donor and the endogenous locus.
Alternatively, the transgene may be inserted into a specific “safe harbor” location in the genome that may either utilize the promoter found at that safe harbor locus, or allow the expressional regulation of the transgene by an exogenous promoter that is fused to the transgene prior to insertion. Several such “safe harbor” loci have been described, including the AAVS1 and CCR5 genes in human cells, and Rosa26 in murine cells (see, e.g., co-owned United States patent applications nos. 20080299580; 20080159996 and 201000218264). As described above, nucleases specific for the safe harbor can be utilized such that the transgene construct is inserted by either HDR- or NHEJ-driven processes.
An especially attractive application of gene therapy involves the treatment of disorders that are either caused by an insufficiency of a secreted gene product or that are treatable by secretion of a therapeutic protein. Such disorders are potentially addressable via delivery of a therapeutic transgene to a modest number of cells, provided that each recipient cell expresses a high level of the therapeutic gene product. In such a scenario, relief from the need for gene delivery to a large number of cells can enable the successful development of gene therapies for otherwise intractable indications. Such applications would require permanent, safe, and very high levels of transgene expression. Thus the development of a safe harbor which exhibits these properties would provide substantial utility in the field of gene therapy.
A considerable number of disorders are either caused by an insufficiency of a secreted gene product or are treatable by secretion of a therapeutic protein. Clotting disorders, for example, are fairly common genetic disorders where factors in the clotting cascade are aberrant in some manner, i.e., lack of expression or production of a mutant protein. Most clotting disorders result in hemophilias such as hemophilia A (factor VIII deficiency), hemophilia B (factor IX deficiency), or hemophilia C (factor XI deficiency). Treatment for these disorders is often related to the severity. For mild hemophilias, treatments can involve therapeutics designed to increase expression of the under-expressed factor, while for more severe hemophilias, therapy involves regular infusion of the missing clotting factor (often 2-3 times a week) to prevent bleeding episodes. Patients with severe hemophilia are often discouraged from participating in many types of sports and must take extra precautions to avoid everyday injuries.
Alpha-1 antitrypsin (A1AT) deficiency is an autosomal recessive disease caused by defective production of alpha 1-antitrypsin which leads to inadequate A1AT levels in the blood and lungs. It can be associated with the development of chronic obstructive pulmonary disease (COPD) and liver disorders. Currently, treatment of the diseases associated with this deficiency can involve infusion of exogenous A1AT and lung or liver transplant.
Lysosomal storage diseases (LSDs) are a group of rare metabolic monogenic diseases characterized by the lack of functional individual lysosomal proteins normally involved in the breakdown of waste lipids, glycoproteins and mucopolysaccharides. These diseases are characterized by a buildup of these compounds in the cell since it is unable to process them for recycling due to the mis-functioning of a specific enzyme. Common examples include Gaucher's (glucocerebrosidase deficiency-gene name: GBA), Fabry's (α galactosidase deficiency—GLA), Hunter's (iduronate-2-sulfatase deficiency—IDS), Hurler's (alpha-L iduronidase deficiency—IDUA), and Niemann-Pick's (sphingomyelin phosphodiesterase 1deficiency—SMPD1) diseases. When grouped together, LSDs have an incidence in the population of about 1 in 7000 births. These diseases have devastating effects on those afflicted with them. They are usually first diagnosed in babies who may have characteristic facial and body growth patterns and may have moderate to severe mental retardation. Treatment options include enzyme replacement therapy (ERT) where the missing enzyme is given to the patient, usually through intravenous injection in large doses. Such treatment is only to treat the symptoms and is not curative, thus the patient must be given repeated dosing of these proteins for the rest of their lives, and potentially may develop neutralizing antibodies to the injected protein. Often these proteins have a short serum half-life, and so the patient must also endure frequent infusions of the protein. For example, Gaucher's disease patients receiving the Cerezyme® product (imiglucerase) must have infusions three times per week. Production and purification of the enzymes is also problematic, and so the treatments are very costly (>$100,000 per year per patient).
Type I diabetes is a disorder in which immune-mediated destruction of pancreatic beta cells results in a profound deficiency of insulin, which is the primary secreted product of these cells. Restoration of baseline insulin levels provide substantial relief from many of the more serious complications of this disorder which can include “macrovascular” complications involving the large vessels: ischemic heart disease (angina and myocardial infarction), stroke and peripheral vascular disease, as well as “microvascular” complications from damage to the small blood vessels. Microvascular complications may include diabetic retinopathy, which affects blood vessel formation in the retina of the eye, and can lead to visual symptoms, reduced vision, and potentially blindness, and diabetic nephropathy, which may involve scarring changes in the kidney tissue, loss of small or progressively larger amounts of protein in the urine, and eventually chronic kidney disease requiring dialysis. Diabetic neuropathy can cause numbness, tingling and pain in the feet and, together with vascular disease in the legs, contributes to the risk of diabetes-related foot problems (such as diabetic foot ulcers) that can be difficult to treat and occasionally require amputation as a result of associated infections.
Antibodies are secreted protein products whose binding plasticity has been exploited for development of a diverse range of therapies. Therapeutic antibodies can be used for neutralization of target proteins that directly cause disease (e.g. VEGF in macular degeneration) as well as highly selective killing of cells whose persistence and replication endanger the hose (e.g. cancer cells, as well as certain immune cells in autoimmune diseases). In such applications, therapeutic antibodies take advantage of the body's normal response to its own antibodies to achieve selective killing, neutralization, or clearance of target proteins or cells bearing the antibody's target antigen. Thus antibody therapy has been widely applied to many human conditions including oncology, rheumatology, transplant, and ocular disease Examples of antibody therapeutics include Lucentis® (Genentech) for the treatment of macular degeneration, Rituxan® (Biogen Idec) for the treatment of Non-Hodgkin lymphoma, and Herceptin® (Genentech) for the treatment of breast cancer. Albumin is a protein that is produced in the liver and secreted into the blood. In humans, serum albumin comprises 60% of the protein found in blood, and its function seems to be to regulate blood volume by regulating the colloid osmotic pressure. It also serves as a carrier for molecules with low solubility, for example lipid soluble hormones, bile salts, free fatty acids, calcium and transferrin. In addition, serum albumin carries therapeutics, including warfarin, phenobutazone, clofibrate and phenytoin. In humans, the albumin locus is highly expressed, resulting in the production of approximately 15 g of albumin protein each day. Albumin has no autocrine function, and there does not appear to be any phenotype associated with monoallelic knockouts and only mild phenotypic observations are found for biallelic knockouts (see Watkins et al (1994) Proc Natl Acad Sci USA 91:9417).
Albumin has also been used when coupled to therapeutic reagents to increase the serum half-life of the therapeutic. For example, Osborn et al (J Pharm Exp Thera (2002) 303(2):540) disclose the pharmacokinetics of a serum albumin-interferon alpha fusion protein and demonstrate that the fusion protein had an approximate 140-fold slower clearance such that the half-life of the fusion was 18-fold longer than for the interferon alpha protein alone. Other examples of therapeutic proteins recently under development that are albumin fusions include Albulin-G™, Cardeva™ and Albugranin™ (Teva Pharmaceutical Industries, fused to Insulin, b-type natriuretic, or GCSF, respectively), Syncria® (GlaxoSmithKline, fused to Glucagon-like peptide-1) and Albuferon α-2B, fused to IFN-alpha (see Current Opinion in Drug Discovery and Development, (2009), vol 12, No. 2. p. 288). In these cases, Albulin-G™, Cardeva™ and Syncria® are all fusion proteins where the albumin is found on the N-terminus of the fusion, while Albugranin™ and Albuferon alpha 2G are fusions where the albumin is on the C-terminus of the fusion.
Thus, there remains a need for additional methods and compositions that can be used to express a desired transgene at a therapeutically relevant level, while avoiding any associated toxicity, and which may limit expression of the transgene to the desired tissue type, for example to treat genetic diseases such as hemophilias, diabetes, lysosomal storage diseases and A1AT deficiency. Additionally, there remains a need for additional methods and compositions to express a desired transgene at a therapeutically relevant level for the treatment of other diseases such as cancers.